The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This invention relates to well cementing operations for oil, gas, water, and geothermal wells and the like. In particular the invention relates to the use of polyvinyl alcohols as an additive to maintain the rheological properties of spacer fluids in such operations.
When an oil well is drilled, a drilling fluid is circulated in the well. The main purpose of the fluid is to lubricate the drilling operations, to control the hydrostatic pressure in the well, and to convey the debris (drill cuttings etc) to the surface and out of the hole. At certain points during the drilling operation, a tubular element or casing is lowered into the drilling hole and cemented by pumping a cement slurry through the casing and into the annular space between the casing and the borehole wall where it is allowed to set. This provides good isolation of the formations through which the borehole passes. Normally a flushing or pre-flushing fluid is pumped through the well before pumping the cement slurry. This flushing fluid, generally referred to as a spacer fluid has two main purposes: to drive out the drilling fluid which is initially situated in the annulus, and to separate the cement slurry and the drilling fluid. It is important to separate the cement slurry and drilling fluid as generally these two fluids are incompatible and mixing of the two can lead to problems, especially in the setting and set properties of the cement. In order to be capable of fulfilling these purposes the fluid present between the drilling mud and cement slurry must maintain stable interfaces between the different fluids and must clean the walls of the borehole before the cement is placed. To obtain a good cement bond the fluid must completely displace the drilling fluid and must remove all residues from the surface of the casing and of the well of the drilling hole, thereby making it possible to achieve good zonal isolation by obtaining good bonding between the cement and the formation and between the cement and the casing.
The presence of a loading agent in the fluid means that it is necessary to add a viscosity-increasing polymer in order to stabilize the suspension particles by creating a yield stress. Designing a spacing fluid consequently consists of finding a minimum concentration of polymer to obtain stable suspensions. In the present application the term “stability” means that the suspension will exhibit no significant sedimentation for at least two hours. This criterion is important in order to avoid the deposition or sedimentation of particles when unexpected stoppages in pumping occur. and the stability is also important in order to ensure that for the period of time which elapses between the mixing of the fluid and its pumping at the surface, no sedimentation occurs in the storage vessel, even in cases where stirring is insufficient. A period of two hours is generally sufficient to result in the necessary stability.
Unfortunately the viscosity of a polymer solution decreases significantly as temperatures increase. As a consequence, to maintain enough viscosity at high temperatures the concentration of polymer has to be increased, however this can result in the fluid exhibiting a high rheological properties at the surface i.e. at ambient temperature. Typical viscosity increasing agents are welan gum, gelan gum, modified guar gum or scleroglucane. Welan gum (Biozan®) is often used to stabilize suspensions is as its rheological properties are less temperature sensitive compared to other polymers. However it still requires at least 30 mins at ambient temperature to be fully hydrated.
Therefore the object of the invention is to provide a viscosity increasing agent for a spacer fluid that will provide constant rheological properties to the spacer fluid during the placement of the fluid down a wellbore.